Hydraulic weft inserter drive in weaving machines

ABSTRACT

The present invention relates to hydraulic weft inserter drives for weaving looms. Each of a pair of hydraulic units is located at opposite ends of a guideway along which the weft inserter is slidably movable. Each hydraulic unit contains a piston whose shaft is extendable in a direction along the guideway in a direction along the guideway on the application of hydraulic pressure on the piston in its normally retracted state. A double pump is operated in synchronism with the loom, each half of the pump being connected to a corresponding hydraulic unit, in such a way that hydraulic pressure is alternately applied to each of the hydraulic units by out of phase operation of each of the pump halves. Biased to retract inside the hydraulic units, the driving pistons of the corresponding hydraulic units are alternately extended, each driving piston being extended in an opposite direction along the guideway, to contact the weft inserter and urge it to move towards the other end of the guideway.

United States Patent 1 Indra et al.

[54] HYDRAULIC WEFTINSERTER DRIVE IN WEAVING MACHINES [75] Inventors:Jaromir Indra; Vaclav Opatril, both of Brno, Czechoslovakia [73]Assignee: Elitex, Zavody Textilniho Strojirenstvi Generalni Rediteislvi,Liberec, Czechoslovakia [22] Filed: July 14, 1971 [21] Appl. No.:162,344

I [30] Foreign Application Priority Data [58] Field of Search ..60/51,52, 53; 139/142, 144

[5 6] References Cited UNITED STATES PATENTS 2,537,895 1/1951 Hicks....l39/144 2,231,331 2/1941 Griffith et al.... ....60/51 3,436,9134/1969 Muller et a1. ....60/51 3,330,305 7/1967 Svaty et al. ..139/144 11 Mar. 27, 1973 Primary Examinerl-lenry S. Jaudon Att0rneyMichael S.Striker 57 ABSTRACT The present invention relates to hydraulic weftinserter drives for weaving looms.

Each of a pair of hydraulic units is located at opposite ends of aguideway along which the weft inserter is slidably movable. Eachhydraulic unit contains a piston whose shaft is extendable in adirection along the guideway in a direction along the guideway on theapplication of hydraulic pressure on the piston in its normallyretracted state. A double pump is operated in synchronism with the loom,each half of the pump being connected to a corresponding hydraulic unit,in such a way that hydraulic pressure is alternately applied to each ofthe hydraulic units by out of phase operation of each-of the pumphalves. Biased to retract inside the hydraulic units, the drivingpistons of the corresponding hydraulic units are alternately extended,each driving piston being extended in an opposite direction along theguideway, to contact the weft inserter and urge it to move towards theother end of the guideway.

11 Claims, 1 Drawing Figure PASTA V0? HYDRAULIC WEFT INSERTER DRIVE INWEAVING MACHINES BACKGROUND OF THE INVENTION 1 Field of the InventionThis invention relates to hydraulic drives, and particularly to ahydraulic weft inserter drive used in a weaving loom.

2. Description of the Prior Art In the past, it has been customary todrive weft inserters either mechanically by means of an inserting arm,or by means of a piston mechanism operating with compressed pair usuallyfed by a twin pump.

In the case where weft inserters are driven by means of compressed air,piston rods are arranged in the two working cylinders mounted at thesides of the weaving machines, said piston rods being provided at theirfront ends with recesses, which are engaged by collets, which are alwaysreleased at a predetermined moment which enable free movement of saidpiston rod, which is acted upon laterally by depressed air.

In both cases, the pertinent device is complicated, noisy duringoperation and shows, moreover, a considerable liability, causingfrequent breakdowns which result in expensive stops of the machine.

SUMMARY OF THE INVENTION The present invention aims at overcoming thedisadvantages of the prior art devices. A hydraulic drive ascontemplated by the present invention results in the reliable picking ofthe weft inserters, the working elements on the extreme positions of thepath along which the weft inserter travels, being hydraulic units. Theuse of a twin hydraulic pump feeding two hydraulic units, one at eachend of the path of travel of the weft inserter, results in a reliableand quiet operation, there being almost no mechanical elements which canbreak down after repeated operations.

In accordance with this invention, a component is slidably mounted formovement in a predetermined path between the two positions along aguideway. A pair of hydraulic units are located at opposite ends of saidpath. Each hydraulic unit consists of a cylinder and a normallyretracted driving piston whose shaft is extensible in a direction alongthe guideway towards the other hydraulic unit. The component is providedwith surfaces to enable abutting contact with the ends of the shafts ofthe driving pistons at each end of the guideway. Although not required,the component moves between the extreme ends of the guideway and comesto rest thereon in preparation for action on it by the respective pistonof a hydraulic unit located on the end where it has come to a stop. Apair of hydraulic pumps are provided, each one of said pumps beingconnected to one of the respective hydraulic units. The pumps areoperated out of phase so that liquid pressure is alternately supplied tothe hydraulic units so that pressure is thereby alternately applied tothe driving piston inside said cylinders. Application of liquid pressureon a cylinder causes it to extend from its normally retracted position,come into abutting contact with one of the surfaces of the componentprovided therefor, and slidably move the component a distance whichcorresponds to the fully extended position of the driving piston.Withthis arrangement, the liquid pressure is removed from the drivingpiston, the driving piston returned to its retracted position and thecomponent comes to a stop along the guideway. With this arrangement, thepump which now feeds the hydraulic unit positioned on the side of theguideway where the component has come to a stop, comes into play andinitiates a liquid pressure buildup in this latter hydraulic unitlikewise causing extension of its respective driving piston. Thecomponent is thereby slidably moved back to its initial position and thereciprocating movement of the component continues so long as the twinpumps are kept in operation.

The novel features which are considered as characteristic of theinvention are set forth in particular in the dependent claim. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows a hydraulic pressaccording to the present invention, partly in a diagrammatic form, showntwo hydraulic units on opposite ends of a guideway and a double pumpfeeding said hydraulic units.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawing, a hydraulic drive mechanism is shown to comprise a pair ofhydraulic units 100, located at opposite ends of a guideway 23 defininga predetermined path. For purposes of specifically describing theinvention, the invention will be described in conjunction with a weavingloom wherein the invention is utilized to shuttle a weft inserter in areciprocating motion along a predetermined path. Thus, component 22will, in this connection, be the weft inserter of the weaving loom.Accordingly, weft inserter 22, to be shuttled in a reciprocating fashionalong guideway 23, moves between two end positions on said predeterminedpath.

A hydraulic unit is situated at each end of the guideway 23. Eachhydraulic unit 100 consists of a body or cylinder 17 which is providedwith a hollow portion or cavity 17. A driving piston 18 is containedwithin the body 17 of hydraulic unit 100, the shaft or piston rod ofsaid driving piston extending through body 17 so as to be slidablymounted thereon. A spring 19, which is mounted on the shaft of drivingpiston 18, rests on one end against the head of driving piston 18 and onthe inside wall of body 17 as shown, so as to urge the driving piston 18to a retracted state into body 17 in the normal state when no otherpressures are applied to said piston 18. Driving piston 18 is capable ofslidable movement with its shaft in a direction opposing the biasing ofspring 19, the shaft of driving piston 18 extending in a directiongenerally along guideway 23. The hydraulic units 100 on both ends ofguideway 23 operate in substantially the same way except that theirrespective pistons move in opposite directions as said pistons move awayfrom their normally retracted positions. Thus, each piston rod projectsalong guideway 23 in a direction towards the other of said hydraulicunits 100 when the restrictive piston moves away from the retractedposition. A component, or the weft inserter in this case, is providedwith surfaces for abutting contact with the ends of the shafts ofpistons 18. Thus, extension of said shafts result in contact with saidsurfaces of said weft inserter and sliding movement of the latter alongsaid guideway as will hereinafter be described.

A twin pump assembly 200 is shown to consist of a body or housing 1 andprovided with two similar hydraulic pumps. Each pump is provided with apiston 2 which is carried by a piston lifter 4. Means 3 are providedbetween said housing 1 and said piston 2 for urging said piston 2 into aretracted state and against piston lifter 4. Each piston lifter 4 is inturn slidably mounted within housing 1 and connected to a roller 5 whichis rotatably mounted within the arms of piston 4 lifter as shown. Roller5, and therefore piston lifter 4, rides on cam means 6 which is rigidlysecured to cam shaft 7. Thus, by rotation of cam shaft 7, cam 6 causespiston lifter 4 to slide up and down within housing 1 in accordance withthe shape of cam 6. It is not critical what means are used to rotate camshaft 7, however, in this application cam shaft 7 is preferablyconnected to the weaving loom which incorporates the weft inserter to bereciprocated. In this connection, and for reasons which will becomeclear in the balance of this disclosure, it is preferred that the camshaft 7 make one revolution for every operating cycle of the loom. Itwill be noted in the FIGURE, that the piston 2 on the left side ofhousing 1 is slightly in extended position while the other piston on theright side of housing 1 is in a more retracted state. As will soonbecome clear, effec tive operation of the hydraulic drive according tothis invention is most effective when the two pistons do not operate insynchronism, that is when the two pistons are both extended or retractedat the same time. In fact, and in connection with a weft inserter for aweaving loom, it is preferred that the pistons 2 move in synchronism butout of phase by difference of 180 so that when one of the pistons 2 isattaining its fully extended position the other of the pistons 2 is justreaching its fully retracted position. This is accomplished by selectingthe shapes and arranging the cams 6 so as to lift rollers 5 in such outof phase relationship.

The pistons 2 are shown to be slidably mounted within cavities 14provided in housing 1. An annular recess 8 is provided near the top ofeach piston 2 so as to provide a narrowed portion. A series of channelsare provided in piston 2 for communicating liquid from working spaces14' on top of pistons 2 and annular recesses 8. This is accomplished byproviding an axial channel which extends from the top of piston 2 downinto the narrowed portion of piston 2 in the vicinity of annular recess8 and a radial channel 9 which is in communication with radial channel10 and extends throughout the narrowed portion of piston 2 and opensinto annular recess 8.

Hydraulic units 100 are connected to the working spaces 14' by means ofpump pipe lines 16 which lead to openings 16' of the hydraulic units100. Cavities 14 are also in liquid communication with areservoir 25which-contains hydraulic fluid. Thus, reservoir pipeline 1] is connectedto vertical inlet channel 12 of housing 1 which is provided between thetwo cavities l4 and extends somewhat beyond the tops of cavities l4.Horizontal channel 13 is provided which is in communication withvertical inlet channel 12 and opens on each side into each of thecavities 14. In this way, hydraulic fluid can be supplied from theliquid reservoir 25 into the individual cavities 14 as required. Withthis arrangement, hydraulic fluid generally flows from reservoir 25 intocavities 14 by way of inlet channel 12 and is forced out by pistons 2through outlet channels 15 where it proceeds through openings 16' intohydraulie units 100. Partly for the reason of providing a closed circuitfor the flow of hydraulic fluid, adjustable throttle valves 20 areprovided on hydraylic units which communicate the interior portions 17'thereof with valve pipelines 21 which lead to liquid reservoir 25.

As just described, throttle valves 20 provide an escape means tohydraulic fluid contained within hollow cavities 17 of hydraulic units100. In addition to allowing pressurized hydraulic fluid to escape,adjustment of throttling nozzle 20 also serves the purpose of regulatingthe necessary stroke of driving piston 18. Thus, by closing up throttlevalve 20, less hydraulic fluid is permitted to escape and the overallstroke or the distance travelled by driving piston 18 during the entireextension step will be small. On the other hand opening up throttlevalve 20 will have the opposite effect. Thus, a larger throttle valveopening will permit more hydraulic fluid to escape through valvepipeline 21 into reservoir 25 This will permit more hydraulic fluid toenter through opening 16', and consequently driving piston 18 willadvance further under these conditions. Another purpose of throttlevalve 20 is that, by admitting a small volume of liquid during eachdischarge into the liquid reservoir, a constant circulation of liquidbetween piston 2 and the cavity 17' in body 17 is made possible, wherebyheating of the compressed liquid is prevented.

A weft inserter 22, therefore, is mounted adjacent to the projectingends of driving pistons 18, said weft inserter being movable alongguideway 23. At each extreme position of weft inserter 22, a device 24for reversing the weft inserter is mounted, said device being of knownconstruction. This known device is not the subject of the presentinvention and therefore not further specified.

The device as specified above operates as follows:

The cam shaft 7 of the pump is driven from a driving mechanism of aweaving loom at such a speed, that during one operating cycle of theloom one revolution of the cam shaft 7 is completed. During rotation ofcam shaft 7, cam 6 contacts roller 5 of the piston lifter 4, piston 2begins to move upwards and pushes the liquid out of working space 14. Inthe first phase of the discharge stroke, the liquid escapes through anopen channel 13 back into the liquid reservoir. Only from the moment atwhich the upper edge of piston 2 covers the upper end of channel 13, theliquid is discharged from the working space through pipeline 16 into thehollow in body 17, where it acts by its pressure upon driving a piston18. Due to pressure of the transferred liquid, driving piston 18 isejected against weft inserter 22 which has imparted motion thereupon.The discharge of the liquid from the working space 14' in the hollowbody 17, and thus the driving motion of driving piston 18, lasts untilthe upper end of the annular recess 8 on piston 2 uncovers channel 13.From that moment, during the stroke of piston 2 until its upper deadcenter the discharged liquid by means of piston 2 escapes from theworking space 14' nearly without resistance through channels and 9 andthrough the annular recess 8 into channel 13 and therefrom throughchannel 12 and the pipeline 11 attached thereto back to the liquidreservoir, whereupon the pressure of liquid in the working space 14' andin the hollow of body 17 rapidly drops, whereupon the driving piston 18is stopped.

In the course of the effective discharge of liquid from the workingspace 14' into the hollow 17' in body 17, a small part of the liquidescapes through throttle valve 20 in pipelines 21 and 11 back into theliquid reservoir. The quantity of liquid thus escaped is determined bythe cross-sectional area of flow of throttle valve 20.

Upon further rotation motion of cam shaft 7, as soon as roller 5 beginsto move along the dependent parts of cam 6, piston 2 together with thepiston lifter 4 is returned downwards by action of the prestressedspring 3. The increasing working space 14' is now being filled by liquidflowing thereto from the hollow of 17' of body 17 due to the backmovement of driving piston 18 caused by the prestressed spring 19 untildriving piston 18 attains its retracted position. Upon further movementof piston 2 in the downward direction, the volume of liquid which hasescaped in the course of the effective discharge stroke through thethrottling valve 20 is replaced, after uncovering the channel 13 by theupper end of piston 2, by liquid fed from the liquid reservoir.

In the manner as specified above, driving pistons 18 impart to the weftinserter 22 such impulses, that said weft inserter moves in areciprocating fashion between positions determined by the distances towhich the driving pistons 18 extend. Once the weft inserter 22 is movedinto a position by one of the driving pistons 18, the motion of the weftinserter 22 is reversed by action of device 24. Shortly afterwards, thecam of the other piston imparts the discharge stroke thereto, whereuponthe driving piston 18 at the opposite end of the machine imparts a backmovement to the weft inserter 22. After arrival of the weft inserter 22into the original position, said weft inserter is again reversed bymeans of device 24 and shortly afterwards, the cam of the first piston 2imparts the discharge stroke thereto and the weft inserter 22 is ejectedagain upon action of driving piston 18 into its other extreme position.This reciprocating motion continues during the entire operation of theloom.

While the invention has been illustrated and described as embodied in ahydraulic drive for weft insertion in weaving looms, it is not intendedto be limited to the details shown, since various modifications andstructural changes may be made without departing in any way from thespirit of the present inventron.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for the standpoint of prior art, fairly constituteessential characteristics of the generic or specific aspects of thisinvention and, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:

l. A hydraulic drive, particularly for weft insertion in weaving looms,comprising a component movable in a predetermined path between twopositions; a pair of hydraulic units respectively located at oppositeends of said path and each comprising a cylinder and normally retractedpiston means movable in the direction towards the other unit formovement of said component in said path; pump means comprising twoindependent pumps for respectively supplying liquid under pressure tosaid cylinders; and drive means for driving said two pumps out of phasewith each other to thereby alternatingly moving said piston means of oneunit towards the other unit so as to move said component between saidpositions thereof.

2. A hydraulic device as defined in claim 1, and including a pair ofuninterrupted pump pipe lines respectively connecting said two pumpswith the respective cylinders of said unit.

3. A hydraulic drive, particularly for weft insertion in a weaving loom,comprising a component movable in a predetermined path between twopositions; a pair of hydraulic units respectively located at oppositeends of said path and each comprising a cylinder and normally retractedpiston means movable in the direction towards the other unit formovement of said component in said path; pump means comprising twoindependent pumps respectively connected to said cylinders of the unitfor alternatingly supplying liquid under pressure into the respectivecylinder to move the associated piston means toward the other unit andthereby said component between the positions thereof, each of said pumpscomprising a pump cylinder, a normally retracted pump piston movable insaid cylinder between two positions one of which is a fully retractedposition in the charging stroke of said pump and the other of which is afully extended position in the discharging stroke of said pump; powermeans for moving said pump pistons in the respective pump cylinders inout of phase relationship with respect to each other; and a pair of pumppipe lines respectively connecting said pumps to said hydraulic units.

4. A hydraulic drive as in claim 3, wherein said component is a weftinserter in a weaving loom and wherein said power means consists of apair of cams, mounted on a rotating weaving loom shaft, the cams beingrotated and rigidly mounted on said shaft so as to be displaced by 180relative to one another, and lifting means supporting said pump pistonsand riding on said cams in the respective pump for following said camsand lifting said pump pistons from their retracted positions.

5. A hydraulic drive as in claim 4, wherein each pump piston completes acharging and a discharging stroke for each 360 revolution of said loomshaft.

6. A hydraulic drive as in claim 4, wherein said loom shaft completes a360 revolution for each operating cycle of the weaving loom.

7. A hydraulic drive as in claim 3, wherein each pump piston is providedin its upper part with a radial annular recess, a radial channel in saidpump piston opening into the annular recess, and an axial channelcommunicating with the radial channel and extending from the radialchannel through the pump piston to the top end of said pump piston andopening in a working space above said pump piston in the associated pumpcylinder.

8. A hydraulic drive as in claim 7, wherein outlet channels are providedfor expelling hydraulic fluid in the discharge stroke of said pumppistons; and pump pipelines, connected to said outlet channels, providedfrom conducting the expelled hydraulic fluid to said hydraulic units.

9. A hydraulic drive as in claim 8, further having a reservoir forhydraulic fluid; and an inlet channel in the side of each of said pumpcylinders, communicating with the working spaces near the tops of saidpump cylinders at points just above the tops of said pump pistons whenthey are in their extreme charging stroke positions, which are connectedto said reservoir.

10. A hydraulic drive as in claim 9, wherein the radial annular recessesare located along said pump pistons below the top surfaces of said pumppistons a distance corresponding to the distance between the top of saidpump cylinders and the points where said inlet channels are introducedinto said pump cylinders, whereby upon movement approaching extremedischarging positions of said pump pistons, said inlet channels areconnected with the upper working space and said outlet channels by meansof said radial and axial channels in said pump pistons.

11. A twin-pump for generating alternating pressures, out of phase by intwo hydraulic systems, comprising an enclosure member having a pair ofidentical cavities; a pair of pistons slidably mounted in said cavitiesfor movement between two positions, biasing means for urging saidpistons into retracted positions; outlet channels connecting the tops ofsaid cavities to said hydraulic systems; a reservoir; inlet channelsconnecting said cavities to said reservoir, said inlet channels beingopened into said cavities at points above said tops of said retractedpistons, said pistons also havingannular recesses at points on saidpistons so that said recesses communicate with said inlet channels inthe piston extended positions, the pistons further having radialchannels which open into said annular recesses, and axial channels whichconnect said radial channels with the tops of said pistons and theoutlet channels whereby fluid communication is obtained between saidoutlet channels and said reservoir in piston extended positions; and camdriving means for moving said pistons, out of phase with one another, tourge said pistons into extended positions.

1. A hydraulic drive, particularly for weft insertion in weaving looms,comprising a component movable in a predetermined path between twopositions; a pair of hydraulic units respectively located at oppositeends of said path and each comprising a cylinder and normally retractedpiston means movable in the direction towards the other unit formovement of said component in said path; pump means comprising twoindependent pumps for respectively supplying liquid under pressure tosaid cylinders; and drive means for driving said two pumps out of phasewith each other to thereby alternatingly moving said piston means of oneunit towards the other unit so as to move said component between saidpositions thereof.
 2. A hydraulic device as defined in claim 1, andincluding a pair of uninterrupted pump pipe lines respectivelyconnecting said two pumps with the respective cylinders of said unit. 3.A hydraulic drive, particularly for weft insertion in a weaving loom,comprising a component movable in a predetermined path between twopositions; a pair of hydraulic units respectively located at oppositeends of said path and each comprising a cylinder and normally retractedpiston means movable in the direction towards the other unit formovement of said component in said path; pump means comprising twoindependent pumps respectively connected to said cylinders of the unitfor alternatingly supplying liquid under pressure into the respectivecylinder to move the associated piston means toward the other unit andthereby said cOmponent between the positions thereof, each of said pumpscomprising a pump cylinder, a normally retracted pump piston movable insaid cylinder between two positions one of which is a fully retractedposition in the charging stroke of said pump and the other of which is afully extended position in the discharging stroke of said pump; powermeans for moving said pump pistons in the respective pump cylinders in180* out of phase relationship with respect to each other; and a pair ofpump pipe lines respectively connecting said pumps to said hydraulicunits.
 4. A hydraulic drive as in claim 3, wherein said component is aweft inserter in a weaving loom and wherein said power means consists ofa pair of cams, mounted on a rotating weaving loom shaft, the cams beingrotated and rigidly mounted on said shaft so as to be displaced by 180*relative to one another, and lifting means supporting said pump pistonsand riding on said cams in the respective pump for following said camsand lifting said pump pistons from their retracted positions.
 5. Ahydraulic drive as in claim 4, wherein each pump piston completes acharging and a discharging stroke for each 360* revolution of said loomshaft.
 6. A hydraulic drive as in claim 4, wherein said loom shaftcompletes a 360* revolution for each operating cycle of the weavingloom.
 7. A hydraulic drive as in claim 3, wherein each pump piston isprovided in its upper part with a radial annular recess, a radialchannel in said pump piston opening into the annular recess, and anaxial channel communicating with the radial channel and extending fromthe radial channel through the pump piston to the top end of said pumppiston and opening in a working space above said pump piston in theassociated pump cylinder.
 8. A hydraulic drive as in claim 7, whereinoutlet channels are provided for expelling hydraulic fluid in thedischarge stroke of said pump pistons; and pump pipelines, connected tosaid outlet channels, provided from conducting the expelled hydraulicfluid to said hydraulic units.
 9. A hydraulic drive as in claim 8,further having a reservoir for hydraulic fluid; and an inlet channel inthe side of each of said pump cylinders, communicating with the workingspaces near the tops of said pump cylinders at points just above thetops of said pump pistons when they are in their extreme charging strokepositions, which are connected to said reservoir.
 10. A hydraulic driveas in claim 9, wherein the radial annular recesses are located alongsaid pump pistons below the top surfaces of said pump pistons a distancecorresponding to the distance between the top of said pump cylinders andthe points where said inlet channels are introduced into said pumpcylinders, whereby upon movement approaching extreme dischargingpositions of said pump pistons, said inlet channels are connected withthe upper working space and said outlet channels by means of said radialand axial channels in said pump pistons.
 11. A twin-pump for generatingalternating pressures, out of phase by 180*, in two hydraulic systems,comprising an enclosure member having a pair of identical cavities; apair of pistons slidably mounted in said cavities for movement betweentwo positions; biasing means for urging said pistons into retractedpositions; outlet channels connecting the tops of said cavities to saidhydraulic systems; a reservoir; inlet channels connecting said cavitiesto said reservoir, said inlet channels being opened into said cavitiesat points above said tops of said retracted pistons, said pistons alsohaving annular recesses at points on said pistons so that said recessescommunicate with said inlet channels in the piston extended positions,the pistons further having radial channels which open into said annularrecesses, and axial channels which connect said radial channels with thetops of said pistons and the outlet channels whereby fluid communicationis obtained Between said outlet channels and said reservoir in pistonextended positions; and cam driving means for moving said pistons, outof phase with one another, to urge said pistons into extended positions.